The present invention relates to an imaging apparatus for obtaining a computed tomogram using a nuclear magnetic resonance phenomenon (a magnetic resonance imaging apparatus, abbreviated as MRI apparatus hereinafter). In particular, it relates to an MRI method and apparatus characterized by its image processing in multi-slab imaging for obtaining images of a plurality of slabs.
As an angiography using an MRI apparatus, there is a Time of Flight method (TOF) which utilizes the fact that signals obtained from in-flow blood in a selectively excited region is enhanced. 3D-TOF is a three dimensional measurement thereof, in which a region having a predetermined thickness (slab) is excited as the selectively excited region and signals encoded both in a phase-encoding direction and a slice direction are measured. In the 3D-TOF method, static portions are saturated by repeated excitation of the same slab, and signals of blood, which flows into the region and has not been excited, are obtained. The 3D-TOF method based on such a principle has the following drawbacks.
(1) In a portion where the blood flow is relatively slow, signals in the selectively excited region may be saturated due to repeated excitation of spins and blood signals at the flow-out side of the region are lowered.
(2) In a portion where the blood flow is relatively fast, the excited site moves during the echo time (TE) from the excitation to acquisition of signals and, therefore, signals of blood that flows into the slab immediately after excitation disappear.
Besides these problems, there is another problem regardless of the blood movement. That is, signals at the opposite sides of a slab are lowered because of the imperfect square shape of a slab exciting profile.
In order to suppress a bad influence upon images provided by the above phenomenon, i.e., degradation of blood extraction caused by disappearance of the blood signal due to multiple excitation and by the blood signal disappearance of fast in-flow blood immediately after excitation, a multi-slab imaging was proposed and becomes a main stream. In the multi-slab imaging, as shown in FIG. 11(a), the region to be imaged (1116, seen in FIG. 11(b)) is divided into a plurality of slabs 1101-1104 that overlap each other, and signals obtained from these slabs are processed to obtain an angiogram of the region. In the image processing, the data at the opposite sides of slab (data portion corresponding to the shaded portion 1100 in the drawing) are deleted from data obtained for each slab and the rest of data are connected to reconstruct an image.
Data 1115 produced by connecting the opposite ends of individual slab data has discontinuity at the connected part of slabs as shown in FIG. 11(b), assuming that a blood vessel has blood signal profiles 1111-1114 for each slab. This discontinuity produces a boundary artifact in a blood image and prevents precise diagnosis.
Therefore, an object of the present invention is to provide an MRI method having an excellent blood imaging ability and capable of eliminating the discontinuity between slabs in the multi-slab imaging. Another object of the present invention is to provide an MRI apparatus suitable for realizing the MRI method of the present invention.
An MRI method according to the present invention performs a three dimensional data measurement of NMR signals successively for a plurality of slabs constituting a desired region of an object and reconstructs images of the desired region of the object by processing the measured data of each slab, wherein a plurality of slabs are selected so that adjacent slabs partially overlap each other and the overlapped data in the measured data of each slab are subjected to weighted addition for each data in the identical slice position. Discontinuity between slabs can be removed by thus weighting and adding the overlapping data. In the MRI method of the present invention, the overlapping degree of 50% or more is particularly effective because data in every slice positions are subjected to addition operation. The overlapping degree of 60-75% is more preferable.
A weight function used in the weighted addition is preferably a slab profile curve or an approximation curve thereof. Data obtained by the weighted addition may be preferably normalized in order to remove the fluctuation of signal values caused accompanying with the weighted addition. A manner of normalization can be changed depending on a slab profile. For example, when the slab profile is square or approximately square, signals are normalized so that the sum of the weight function values becomes constant. When the signal intensities decrease toward opposite sides of the slab profile, for data in a slice position where the sum of the weight function is a constant value S or more, signals are normalized so that the sum W of the weight function becomes the constant value S, and for data in a slice position where the sum W is not more than the constant S, signals are normalized to have values between the weight function value assigned to the data and the constant S. Increase of noise in data at the opposite sides can be suppressed by performing such a normalization processing.
In the MRI method of the present invention, the three dimensional measurement typically includes a blood imaging sequence based on the Time-of-Flight method.
An MRI apparatus according to the present invention comprises magnetic field generating means for generating a static magnetic field, RF magnetic field and gradient magnetic field in a space where an object to be examined is placed, detecting means for detecting NMR signals emitted from the object, signal processing means for reconstructing images of a desired region of the object using the NMR signals, display means for displaying the images, control means for controlling the magnetic filed generating means, detecting means and signal processing means according to a predetermined imaging sequence, means for setting imaging conditions, and being equipped with a multi-slab measuring sequence for repeatedly performing a three dimensional measurement of slabs of the object as the imaging sequence, wherein the setting means comprises means for setting parameters for the multi-slab measurement, and the signal processing means comprises means for performing a weighted addition of data of the overlapping slab portion for each slice position and means for determining a weight function used in the weighted addition based on parameters set by the setting means.
The setting means includes means for setting a number of slabs, a number of slices constituting one slab, and the overlapping degree of slabs as parameters. The MRI apparatus facilitates performance of the MRI method of the present invention.